Selenites

Selenites.—The crystal structure of selenious acid, H2Se03, was solved by X-rays in 1949. It has been further resolved by a precise ND study. Pyramidal SeOs units have Se-O [1.643(2) A] shorter than the pair of Se-OH bonds [1.735(2) and 1.743(2) A]. The oxygen atom of the first accepts two hydrogen bonds, the latter two donate their hydrogens. The SeOa units are linked into infinite double layers by these hydrogen bonds. These are two 

Lithium hydrogen selenite has been prepared in 95% yield by neutralization of a selenious acid solution with LiOH to pH 4—4.5. 

Shipilova and T. V. Revzina, Metody. Poluch. Khim. Reactiv. Prep., 1972, No. 

446 Tellgren, D. Ahmad, and R. Limingen, J. Solid-State Chem., 1973, 6, 250. 

The thermal behaviour of NaHSeOs in neutral atmospheres has been studied and shown to follow the sequence  

The dehydration and dissociation of BeS03,4H20 has been investigated. Loss of water takes place in two stages at 120 and 260 C crystallization of the salt at 340 °C is followed at 485 C by decomposition, with the release of SeOg into the gas phase and the formation of BeO as the final solid phase  

Spectroscopic and magnetic measurements on the compound VO-(02Se0Et)2, prepared by the reaction of VOCI2 with H2Se03 in EtOH, suggest the monomeric structure (44). Pyrolysis of the compound proceeds by a com- 

Antimony compounds are based on antimony, an element that exhibits both metal and nonmetal properties. Many of its compounds are toxic and corrosive, particularly the soluble salts. They include antimony iodide and antimony perchloride. Some antimony compounds decompose in water to produce toxic gases e.g., antimony sulphate decomposes to sulphur dioxide while antimony bromide produces bromine gas. 

Arsenic compounds are based on the metalloid arsenic. They are extremely toxic and include the arsenates, those that contain the AsO/ group 

Tellurium compounds are toxic. They include tellurium sulphide and tellurium dioxide. 

Dissociation of many inorganic acids yields a variety of salts. 

Chlorine forms a number of ions with oxygen, all of which are strong oxidizing agents. They react similarly to bromates, evolving chlorine gas in contact with acids. They include 

The brick red Yb202Se forms on heating YbSe in the air at 580 to 670°C, with Yb203 nSe02 (n 1) as an intermediate as shown by DTA, TG, thermogravimetric, and X-ray investigations. The material becomes beige colored at 760°C, Semenov-Kobzar, Nikol skaya [1]. 

This section deals with the neutral selenites, hydrogenselenites, and basic selenites, the corresponding solution systems. 

The selenites are usually prepared by precipitation in aqueous solutions of H2Se03 or Na2Se03 and a salt of M (such as the chloride) as amorphous products containing indeterminate amounts of water. On heating from -60 to 165°C they begin to lose H2O and crystallize later on. Well-defined hydrates are occasionally formed as intermediates. Sometimes dehydration takes place together with decomposition. See, for instance, Perkovskaya [2], Maier et al. [3]. 

The solubility S in H2O at 20 0.05°C is determined (possibly on hydrated samples) after shaking for 6 h  

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Phytoremediation is also being developed for dealing with soils contaminated with high levels of selenium in California again B.juncea seems to be particularly effective in accumulating the contaminant from soil, and all plants tested were more effective at removing selenate than selenite (92). This is an interesting contrast to bacterial systems, where selenite reduction is more commonly found than selenate reduction. 

The Reaction. Acrolein has been produced commercially since 1938. The first commercial processes were based on the vapor-phase condensation of acetaldehyde and formaldehyde (1). In the 1940s a series of catalyst developments based on cuprous oxide and cupric selenites led to a vapor-phase propylene oxidation route to acrolein (7,8). In 1959 Shell was the first to commercialize this propylene oxidation to acrolein process. These early propylene oxidation catalysts were capable of only low per pass propylene conversions (ca 15%) and therefore required significant recycle of unreacted propylene (9—11). 

Thousands of compounds of the actinide elements have been prepared, and the properties of some of the important binary compounds are summarized in Table 8 (13,17,18,22). The binary compounds with carbon, boron, nitrogen, siUcon, and sulfur are not included these are of interest, however, because of their stabiUty at high temperatures. A large number of ternary compounds, including numerous oxyhaUdes, and more compHcated compounds have been synthesized and characterized. These include many intermediate (nonstoichiometric) oxides, and besides the nitrates, sulfates, peroxides, and carbonates, compounds such as phosphates, arsenates, cyanides, cyanates, thiocyanates, selenocyanates, sulfites, selenates, selenites, teUurates, tellurites, selenides, and teUurides. 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

In 1956 selenium was identified (123) as an essential micronutrient iu nutrition. In conjunction with vitamin E, selenium is effective iu the prevention of muscular dystrophy iu animals. Sodium selenite is adrninistered to prevent exudative diathesis iu chicks, a condition iu which fluid leaks out of the tissues white muscle disease iu sheep and infertility iu ewes (see Eeed ADDITIVES). Selenium lessens the iacidence of pneumonia iu lambs and of premature, weak, and stillborn calves controls hepatosis dietetica iu pigs and decreases muscular inflammation iu horses. White muscle disease, widespread iu sheep and cattle of the selenium-deficient areas of New Zealand and the United States, is insignificant iu high selenium soil areas. The supplementation of animal feeds with selenium was approved by the U.S. EDA iu 1974 (see Eeed additives). Much of selenium s metaboHc activity results from its involvement iu the selenoproteia enzyme, glutathione peroxidase. 

Sodium selenate has been used on a small scale in commercial greenhouses, primarily for growing carnations and chrysanthemums. It is transformed by the plants into volatile selenides, which repel red spiders, mites, thrips, and aphids (see Insect control technology). Sodium selenite is not intended for crops which could ultimately be used as food for humans or domestic animals. 

Sodium selenite has also been incorporated into styrene—butadiene mbber and used in a pellet form which results in the slow release of selenium into water. These pellets have been placed in lakes in Sweden which have fish contaminated with mercury owing to high levels of that element in the water. The selenium released by the pellets reacts with mercury to form insoluble, heavy mercury selenide which setties to the lake bottom and removes mercury from the ecosystem (126). 

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